Draw and compare biological specimens
Make proportionate plan and cell drawings, and compare specimens using features that are directly observable.
Section Paper 3 · Advanced practical skills
Cambridge International Biology 9700, 2028–2030. Paper 3 assesses practical skills in a supervised laboratory; these lessons support preparation but do not replace safe, supervised laboratory experience.
Before you begin
- Use biological vocabulary and basic calculations from the AS Biology course. Follow your school laboratory rules and teacher instructions for all practical work.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is the main purpose of a plan diagram?
What you will learn
- Choose between a plan diagram and a cell drawing.
- Record clear shape, relative size and observable contents without shading.
- Compare specimens with paired, observable similarities and differences.
Choose the right drawing
A plan diagram shows the distribution and relative proportions of tissues; it does not show individual cells. A cell drawing shows the observable outline, shape and contents of cells at greater detail.
Use clear, continuous lines and enough space to show the observed features. Do not add details that are not visible. In a plan diagram, preserve relative layer thicknesses and arrangement.
Build a useful comparison
Compare like with like: name an observable feature in both specimens and state how it differs or remains similar. Avoid assigning function, identity or cause unless evidence supports it.
A labelled drawing should point clearly to the structure described. An accurate drawing is based on what is visible in the specimen or image, not a memorised idealised diagram.
Pause and explain
Which comparison is supported by direct observation?
Worked example
Two leaf sections are provided. What should a plan diagram show, and what should be omitted?
Show the worked solution
- Show the distribution and relative widths of tissue layers.
- Use outlines for the layers and retain their order.
- Do not draw individual cells or shade the plan.
Answer Show the tissue layers in the correct order and relative proportions, using clear outlines. Omit individual cells and shading.
From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A section has three visible tissue layers. State four features of a good plan diagram.
Give me a hint
Focus on arrangement, proportions and clean lines.
Compare my reasoning
- Place all three layers in their observed order.
- Keep their relative widths proportionate.
- Use clear continuous lines, with no cells or shading.
Show the three layers in the correct order and relative proportions, using clear unbroken outlines; do not add cells or shading.
Look for these in your work
- Correct tissue arrangement
- Relative proportions preserved
- No cell detail or shading
Specimen A has 12 visible vascular bundles and specimen B has 8 in comparable fields. Write a cautious comparison.
Give me a hint
Name the field context and avoid inferring function.
Compare my reasoning
- State the observed counts.
- Make the comparison in comparable sampled areas.
- Do not infer transport capacity from counts alone.
In the comparable fields observed, specimen A has four more visible vascular bundles than specimen B (12 compared with 8); this count alone does not establish their transport capacity.
Look for these in your work
- Counts compared
- Scope of evidence limited
A drawing labels a structure that cannot be distinguished in the provided image. What should change?
Give me a hint
Use evidence from the image only.
Compare my reasoning
- Remove or qualify the unsupported label.
- Retain labels only for visible features.
- If uncertain, state that the feature cannot be resolved.
Remove the unsupported label or state that the feature cannot be resolved; label only visible structures.
Look for these in your work
- Unsupported feature removed
- Observation limits acknowledged
Common mistakes
- Drawing cells in a plan diagram.
- Adding familiar structures that are not visible.
- Writing “specimen A is better developed” without an observable comparison.
What can you explain now?
What is the main purpose of a plan diagram?
Compare with the explanation
Show tissue distribution and relative proportions
Plan diagrams summarise the arrangement of tissues.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
What belongs in a plan diagram, and how is that different from a cell drawing?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.